1. Field of the Invention
The present invention relates to a receiving apparatus and a receiving method and, more particularly, to a receiving apparatus and a receiving method based on an OFDM method.
2. Description of the Related Art
Modulation methods called orthogonal frequency division multiplexing (OFDM) have recently been proposed as a method for transmitting a digital signal. In an OFDM method, a number of subcarriers orthogonal to each other are provided in a transmission band, data items are respectively assigned to the amplitudes and phases of the subcarriers, and digital modulation is performed by phase shift keying (PSK) or quadrature amplitude modulation (QAM). This method uses a reduced band for one subcarrier since the transmission band is divided with respect to a number of subcarriers, so that the modulation speed is reduced. This method, however, achieves the same total transmission speed as other conventional modulation methods because the number of carriers is large.
In this OFDM method, the symbol speed is reduced since a number of subcarriers are transmitted parallel to each other, so that a multipath period relative to the length of a symbol with respect to time can be reduced. Thus, an OFDM method can be expected as a method ensuring high resistance to multipath interference.
Because of the above-described feature, OFDM methods have attracted attention particularly with respect to transmission of digital ground wave signals susceptible to the influence of multipath interference. For example, Digital Video Broadcasting-Terrestrial (DVB-T) is well known as such digital signal transmission by ground waves.
With the recent progress of the semiconductor technology, it has become possible to achieve discrete Fourier transform (hereinafter referred to as FFT (fast Fourier transform)) and discrete inverse Fourier transform (hereinafter referred to as IFFT (inverse fast Fourier transform)) by hardware. If these transforms are used, modulation and demodulation in accordance with an OFDM method can easily be performed. This has also contributed to the increase of attention to OFDM methods.
FIG. 10 is a block diagram showing the configuration of an example of an OFDM receiver. A receiving antenna 101 captures an RF signal. A multiplication circuit 102 calculates the product of the RF signal and a signal which is output from a tuner 103 and which has a predetermined frequency. A bandpass filter 104 extracts the desired IF signal from an output from the multiplication circuit 102. An A/D conversion circuit 105 converts the IF signal extracted by the bandpass filter 104 into a digital signal.
A demultiplexer 106 separates and extracts an I channel signal and a Q channel signal from the digitized IF signal. Lowpass filters 107 and 108 respectively convert the I channel signal and the Q channel signal into baseband signals by removing unnecessary high-frequency components contained in the I channel signal and the Q channel signal.
A complex multiplication circuit 109 removes a carrier frequency error in the baseband signals by a signal of a predetermined frequency supplied from a numerical control oscillation circuit 110, and thereafter supplies the baseband signals to a fast Fourier transform circuit 112, which frequency-decomposes the OFDM time signals to form I and Q channel received data.
A correlation value calculation circuit 113 obtains a correlation value of the OFDM time signal converted into the base band and the OFDM signal delayed by an effective symbol period, and makes the fast Fourier transform circuit 112 start calculating when the correlation value becomes maximized.
A carrier frequency error calculation circuit 114 calculates a carrier frequency error by detecting a frequency power deviation and outputs the calculation result to an addition circuit 111. The addition circuit 111 calculates the sum of the outputs from the carrier frequency error calculation circuit 114 and the correlation value calculation circuit 113 and outputs the calculation result to the numerical control oscillation circuit 110.
A clock frequency reproduction circuit 115 forms a control signal by referring to the I channel data and Q channel data to control the frequency of oscillation of the clock oscillation circuit 116. The clock oscillation circuit 116 forms and outputs a clock signal in accordance with the control signal supplied from the clock frequency reproduction circuit 115.
The operation of the above-described example of the conventional apparatus will next be described.
The multiplication circuit 102 calculates the product of an RF signal captured by the receiving antenna 101 and the signal supplied from the tuner 103 and having a predetermined frequency. The signal output from the multiplication circuit 102 is supplied to the bandpass filter 104, which extracts the IF signal.
The A/D conversion circuit 105 converts the IF signal output from the bandpass filter 104 into a digital signal and supplies the digital signal to the demultiplexer 106. The demultiplexer 106 separates and extracts an I channel signal and a Q channel signal from the digitized signal and supplies these signals to the lowpass filters 107 and 108. The lowpass filters 107 and 108 respectively convert the I channel signal and the Q channel signal into baseband signals by removing aliasing components which are unnecessary high-frequency components contained in the I channel signal and the Q channel signal.
The complex multiplication circuit 109 removes a carrier frequency error in the baseband signals by a signal of a predetermined frequency supplied from the numerical control oscillation circuit 110, and thereafter supplies the baseband signals to the fast Fourier transform circuit 112. The fast Fourier transform circuit 112 frequency-decomposes the OFDM time signal to form I and Q channel received data.
The correlation value calculation circuit 113 calculates a value representing a correlation between the OFDM time signal converted into the base band and the OFDM signal delayed by the effective symbol period and makes the fast Fourier transform circuit 112 start calculating when the correlation value becomes maximized. Consequently, the fast Fourier transform circuit 112 can accurately extract data contained in the I channel signal and Q channel signal sent from the transmitting side.
The correlation value calculation circuit 113 is arranged to control the numerical control oscillation circuit 110 in cooperation with the carrier frequency error calculation circuit 114. Details of the configurations and the operations of these circuits will now be described.
In the example shown in FIG. 10, a carrier frequency error is detected by being decomposed into a component smaller than the subcarrier frequency interval and a component of a unit frequency corresponding to the subcarrier frequency interval. That is, the correlation value calculation circuit 113 calculates a carrier frequency error up to .+-.1/2 of the subcarrier frequency interval of the OFDM signal by calculating the correlation value by utilizing the periodicity of the OFDM time signal waveform. Also, the carrier frequency error calculation circuit 114 obtains a frequency error of a unit frequency corresponding to the subcarrier frequency interval by calculating electric power of the frequency components of the OFDM signal output from the fast Fourier transform circuit 112. The sum of the outputs of the correlation value calculation circuit 113 and the carrier frequency error calculation circuit 114 is calculated by the addition circuit 111, and the frequency of oscillation of the numerical control oscillation circuit 110 is controlled according to the calculated value (error value).
FIG. 11 is a diagram showing details of an example of the carrier frequency error calculation circuit 114 shown in FIG. 10. Squaring circuits 203 and 204 shown in FIG. 11 are supplied with I channel signal 201 and Q channel signal 202, square these signals, and output the squares (the values corresponding to the electric power of carriers). An addition circuit 205 calculates the sum of the outputs of the squaring circuits 203 and 204 and outputs the calculation result. A difference calculation circuit 206 divides the signal corresponding to the power of the frequencies output from the addition circuit 205 into two regions (regions A and B), calculates the total power in each region, subtracts the power in one region (region A) from that in the other region (region B), and outputs the subtraction result. An averaging circuit 207 sums up the difference values output from the difference calculation circuit 206 with respect to several symbols, divides the sum by the number of symbols, and outputs the average of the differences.
The operation of this example will next be described with reference to FIG. 12A to 12C.
FIG. 12A shows placement (spectrum) of signals output from the fast Fourier transform circuit 112 with respect to frequencies in a situation where the frequency of oscillation of the numerical control oscillation circuit 110 (reproducing carrier frequencies) is correctly set. As shown in this diagram, data processed by N-points fast Fourier transform is formed of 0 to (N-1) subcarriers. If two regions on the opposite sides of a center N/2 are A and B, N/2 subcarriers are contained in each region.
As shown in FIG. 12A, in the case where the frequencies of reproducing carriers are correctly controlled, the numbers of subcarriers respectively placed in the regions A and B are equal to each other, so that the powers of subcarriers in the regions A and B are equal to each other. FIG. 12B shows placement of signals output from the fast Fourier transform circuit 112 with respect to frequencies in a situation where signals in the base band are demodulated with reproducing carrier frequencies having a frequency error corresponding to one subcarrier frequency interval (see FIG. 4A). In this example, the numbers of subcarriers respectively placed in the regions A and B are not equal to each other, so that the powers in the two regions are different.
In the conventional art shown in FIG. 11, a reproducing carrier frequency error is detected based on the above-described principle.
That is, the squaring circuits 203 and 204 respectively square the I channel signal and the Q channel signal to obtain the powers of these signals. The addition circuit 205 calculates the sum of the outputs of the squaring circuits 203 and 204 and outputs the calculation result to the difference calculation circuit 206. The difference calculation circuit 206 outputs the power value obtained by subtracting the total power of subcarriers in the region B from the total power of subcarriers in the region A (the difference between the powers in the regions A and B) in the power of subcarriers output from the addition circuit 205.
If reproducing is performed with correct reproducing carrier frequencies as shown in FIG. 12A, the powers of subcarriers in the regions A and B are equal to each other and, accordingly, the output of the differential calculation circuit 206 is zero. On the other hand, if reproducing carrier frequencies have an error corresponding to one subcarrier frequency interval as shown in FIG. 12B), the difference between the powers of subcarriers in the regions A and B corresponds to one subcarrier component.
The average circuit 207 obtains and outputs the average (average of power differences) 208 of outputs from the difference calculation circuit 206 corresponding to several symbols in order to eliminate the influence of noise or the like contained in the signals. The numerical control oscillation circuit 110 changes the oscillating frequency according to the output of the average circuit 207. Consequently, the oscillating frequency of the numerical control oscillation circuit 110 is maintained at a predetermined frequency by the feedback loop formed by the carrier frequency error calculation circuit 114 and the correlation value calculation circuit 113.
The method of dividing subcarriers pertaining to one symbol period in two regions and obtaining an error in reproducing carrier frequencies from the difference between the powers thereof ensures that, if, for example, noise uniform in power through the entire frequency band is mixed in the received signal, the influence of noise will be removed by averaging. However, if noise not uniform with respect to frequencies is mixed due to multipath interference or the like as shown in FIG. 12C, the power balancing itself is variable, so that an error in reproducing carrier frequencies cannot be detected.